I have a Premium Workspace Email plan, but I cannot access the accounts in it via IMAP.
Seems to indicate that the only accounts which do not support IMAP are Free ones. This is definitely a paid plan.
How do I get IMAP access for this plan?
You're premium workspace email plan does support IMAP. If you're still having issues with this, you really need to contact our customer support team as they can look into your account and help you with what's causing the problem.
You can contact our team for phone support 24/7 and if you're not in the United States, you can call local numbers to your region that are available here.
Chat is currently available 5am to 6:30pm Arizona time Monday-Friday. Please access GoDaddy chat by clicking on the "Product Support" link at the bottom of our United States homepage. You'll probably have to manually change the region you're located in to the United States. The chat button may not appear if the chat queue is super busy. You can leave your browser open to the chat page and the chat button will reappear when the queue becomes available. Chat hours are available Monday through Friday from 5AM to 6PM (Arizona, USA time zone: GMT-7:00).
All the best!